According to her chief economic advisor, Gene Sperling, Hillary Clinton is really serious about reopening the North American Free Trade Agreement if she becomes president. Her anti-NAFTA rhetoric of late is not just campaign blarney.

While in Ottawa on Thursday Mr. Sperling, who is also a staff member of the Council on Foreign Relations and a fellow at the Center for American Progress, told an audience that Ms. Clinton "has made very clear that she would reopen and renegotiate the agreement." And if she failed to achieve a better deal for American workers, Mr. Sperling explained, Ms. Clinton would be prepared to end the trade treaty.

Frankly, we are no more convinced of the former first lady's sincerity now than when she and her chief rival, Barack Obama, first made NAFTA an issue in their nomination race a month ago in Ohio. With a crucial primary coming up in Pennsylvania next month, and with Mrs. Clinton desperate to win to remain in the race, it is only natural she would ratchet up her oratory in the union-heavy state and dispatch her campaign minions to underline her conviction on the issue.

Still, campaign bravado can develop a life of its own. A candidate's message can create expectations among his or her supporters that have to be satisfied once he or she has achieved office. So whether or not Mrs. Clinton truly is sincere in her threats against NAFTA, Canadians cannot afford to shrug off her warning. The treaty is simply too integral to our prosperity to take anything about it for granted.

It's a fallacy, perpetrated by unionists, anti-trade activists and, of course, grandstanding politicians, that NAFTA is behind the precipitous decline in well-paying manufacturing jobs here and in the U.S. Indeed, assembly-line employment in Ohio -- where NAFTA was first raised as an issue -- increased thanks to NAFTA during the deal's first three years. It was only after the American dollar appreciated in the late-1990s that jobs began leaving the U.S. --mostly to low-wage countries such as China and India, not Canada.

Under NAFTA, our exports to the U.S. have nearly tripled, while theirs to us have almost doubled. The deal has been good for both countries. Almost two-thirds of Canada's economic growth since the mid-1990s can be traced back to the easier access NAFTA has given us to the vast American market.

If we are to sustain our recent prosperity we must also defend NAFTA.

If either Mr. Obama or Ms. Clinton win the White House this November and carry out their threat to renegotiate the deal, Canada must be prepared to play the oil card. Despite the impression that America is most dependent on Middle Eastern oil, Canada is far and away the Americans' largest foreign supplier of oil, providing almost 25% more oil daily than Saudi Arabia. And, 85% or more of foreign natural gas in the U.S. is from Canada.

American dependence on Canadian energy gives us a trump card to counter any threat from a new administration to derail NAFTA.

It is not yet time, though, to play hardball. For now, Ottawa should concentrate on gently making American legislators and voters aware that good ole reliable, stable, friendly Canada is their #1 energy partner.

That way, if an anti-NAFTA Democrat wins the presidency next fall, she or he will have a harder time painting Canada as a threat to Americans' lifestyle and jobs.